(1) Field of the Invention
The present invention relates to a method for synthesizing boron nitride of the cubic system (CBN) at a high temperature under a high pressure, wherein boron nitride crystals of the cubic system are grown by using their particles as seeds.
(2) Description of the Prior Art
When boron nitride of the cubic system is synthesized according to the hydrostatic pressure method, to obtain crystals having a low inclusion content and a good shape, it is important to grow the crystals at temperatures and pressures very close to the phase equilibrium line. Accordingly, the use of granular seeds is considered effective for controlling the number of nuclei acting as the centers of growth of the crystals.
In synthesizing boron nitride of the cubic system according to the hydrostatic pressure method, a temperature difference growth method and a film growth method are adopted as means for growing the crystals. According to the former method, a seed and a starting material source are made present and a solvent material having a temperature gradient is interposed therebetween in such a manner that the seed is positioned on the low temperature side of the solvent material, where a cubic boron nitride crystal is grown. According to the latter method, low pressure phase boron nitride (i.e., boron nitride of the hexagonal system) is dissolved in a thin film of a solvent material adhering to the periphery of a boron nitride seed particle of the cubic system, and a boron nitride crystal of the cubic system is grown by utilizing the solubility difference between the low pressure phase boron nitride of the hexagonal system and the boron nitride of the cubic system. In the latter method, the crystal size depends principally on the time necessary for the synthesis, the amount of cubic nuclei generated, and the amount of starting boron nitride supplied. If seeds are used for the generation of nuclei, the number of nuclei can be controlled, but because of irregular distribution of the nuclei, deviations of the nucleus-growing time among various points in the sample, and interference among nuclei or growing particles, it is difficult to obtain boron nitride crystals of the cubic system with a uniform particle size in a high yield.
Even if the number of nuclei can be controlled by using seeds, when crystals are grown in a region where the degree of supersaturation of the boron nitride concentration is high, good quality crystals cannot be obtained and the crystal shape is unsatisfactory. Furthermore, the amounts of included impurities such as the solvent material, starting material, and bubbles are increased. In order to reduce the degree of supersaturation, it is necessary to maintain the temperature and pressure conditions in close proximity to the phase equilibrium line. However, in an industrial super-high pressure synthesis apparatus, it is very difficult to regularly measure the temperature and pressure in the reaction zone and to control them to the intended values.
A method is known in which a solvent material and a starting boron nitride are mixed with seeds, the mixture is molded into columns, and the columns are charged in a high pressure apparatus where boron nitride crystals of the cubic system are grown. However, since it is difficult to disperse the seeds uniformly in the mixture, deviations are readily caused in the growth of the crystals.